mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Systemic RNAi in C. elegans
RNA interference, or RNAi, is a biological phenomenon in which short strands of RNA molecules are able to bind to messenger RNA (mRNA) molecules in an antisense, hybridization manner, and prevent them from being translated into their protein products. This process is quite literally one RNA molecule interfering with another, hence the term RNAi. The process of RNAi occurs when double-stranded RNA molecules known as short interfering RNA (siRNA) bind to complementary mRNA and subsequently recruit helper proteins, forming what is known as the RNA-induced silencing complex (RISC). The protein component of this complex has enzymatic activity that functions to cleave the chemical bonds of the mRNA, and thus the mRNA is destroyed and rendered incapable of being translated into protein. RNAi can also be mediated by short, single-stranded RNA molecules known as microRNAs (miRNA) 1, 2, 3. RNAi is known to occur naturally throughout a wide range of organisms from bacteria, to plants, and even humans. It is hypothesized that the RNAi mechanism evolved to enhance a cell's control over gene expression, especially during embryogenesis. Furthermore, it is also thought that RNAi may serve as a defense mechanism against viruses that try to insert their genetic material into host cells. The phenomenon of RNAi was discovered by two scientists, Dr. Andrew Fire and Dr. Craig Mello, who in 1998 demonstrated that when the nematode C. elegans was injected with short double-stranded RNA molecules that had sequences complementary to specific mRNA molecules, the proteins derived from those mRNAs ceased to be produced. Drs. Andrew Fire and Craig Mello shared the 2006 Nobel Prize in Physiology or Medicine for their discovery of RNA interference. Currently, scientists are researching disease treatments based RNAi, because as a molecular biology tool this mechanism offers the potential to effectively silence any gene, or any part of a gene, in the human body. It has already been successfully demonstrated that synthetic small interfering RNA (siRNA) can effectively shut off genes when introduced into human cells. This technology holds the potential to treat a plethora of genetic diseases. However, much research still needs to be done so that scientists can figure out how to effectively and efficiently deliver small RNA molecules to desired locations in the body while minimizing the potential of any unintended side effects. Systemic RNAi in C. elegant Requires the Putative Transmembrane Protein SID-1 One of the most bizarre aspects of RNAi in C. elegans is that it is systemic. It has been shown that when gene-specific double-stranded RNA (dsRNA) is injected into one tissue it leads to the post-transcriptional gene silencing (PTGS) of that gene in other tissues and in the worm’s offspring. Systemic RNAi can be induced in Fig 1 RNAi.jpg Fig 2 RNAi.jpg Fig 3 RNAi.jpg Fig 4 RNAi.jpg C. elegans by directly soaking them in dsRNA, or by feeding them bacteria that express dsRNA. Even though systemic RNAi has yet to be observed in other animals, systemic posttranscriptional gene silencing (PTGS) effects have long been reported in plants 3. The authors of this study sought to determine how the gene-silencing interfering RNAs are transmitted between cells. In order to do this, they created a strain of C. elegans that allows for direct visualization of systemic RNAi. They were able to use this strain to identify systemic RNA interference deficient (sid) gene loci that are required for the spread of gene-silencing information between cells and tissues, but are not required to either initiate or support the RNAi gene-silencing response. SID-1, one of the loci they identified, encodes for a conserved protein that has predicted transmembrane domains. The authors show that SID-1 is expressed in RNAi-sensitive cells, is localized to periphery of these cells, and is required for systemic RNAi 3. Conclusions The results of this study suggest that SID-1 acts as a channel for dsRNA, siRNAs, or some other as of yet undiscovered RNAi signal. The authors' finding also suggest that perhaps SID-1 functions as a receptor and is required for endocytosis of the systemic RNAi signal. The authors note that they were able to detect SID-1::GFP in most non-neuronal cells, and that this observation is consistent with previous findings that neuronal cells tend to be resistant to systemic RNAi. Furthermore, the authors noted that the non-neuronal cells that exhibited the strongest SID-1::GFP levels were those that are directly exposed to the external environment. This suggests that SID-1 may be involved in responding to and interpreting environmental signals such as those from viruses and microbial pathogens. The authors state that SID-1 has a predicted structure similar to human and mouse homologous proteins, suggesting that systemic RNAi may occur in mammals by a similar mechanism that is observed in C. elegans3. References 1. Fire A, et al. (1998). [http://www.nature.com/nature/journal/v391/n6669/full/391806a0.html Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans]. Nature 391:806-811. 2. Hunter CP, et al. (2006). [http://symposium.cshlp.org/content/71/95.long Systemic RNAi in Caenorhabditis elegans]. Cold Spring Harb. Symp. Quant. Biol. 71: 95-100. 3. Winston WM, et al. (2002). [http://www.sciencemag.org/content/295/5564/2456.long Systemic RNAi in C. elegans '' requires the putative transmembrane protein SID-1]. ''Science. 295:2456-2459.